One technology for after-treatment of engine exhaust utilizes selective catalytic reduction (SCR) to enable certain chemical reactions to occur between NOx in the exhaust and ammonia (NH3). NH3 is introduced into an engine exhaust system upstream of an SCR catalyst by injecting urea into an exhaust pathway, or is generated in an upstream catalyst. The urea entropically decomposes to NH3 under high temperature conditions. The SCR facilitates the reaction between NH3 and NOx to convert NOx into nitrogen (N2) and water (H2O). However, as recognized by the inventor herein, issues may arise upon injecting urea into the exhaust pathway. In one example, urea may be poorly mixed into the exhaust flow (e.g., a first portion of exhaust flow has a higher concentration of urea than a second portion of exhaust flow) which may lead to poor coating of the SCR and poor reactivity between emissions (e.g., NOx) and the SCR. Additionally, overly mixing and agitating the urea in the exhaust can likewise cause issues, such as increased deposits.
Attempts to address insufficient mixing include introducing a mixing device downstream of a urea injector and upstream of the SCR such that the exhaust flow may be more homogenous. Other attempts to address urea mixing include a stationary mixing apparatus. One example approach is shown by Cho et al. in U.S. 2013/0104531. Therein, a static mixer is located in an exhaust passage downstream of an external tube for injecting urea. The exhaust gas flows through the exhaust passage and combines with a urea injection before flowing through the static mixer.
However, the inventors herein have recognized potential issues with such systems. As one example, the static mixer described above presents limited mixing capabilities due to a directionality of exhaust outflow through the mixer unable to fully mix a laminar exhaust flow. The static mixer inside the exhaust passage also presents manufacturing and packaging constraints. Varying exhaust passage geometries demand an alteration in the manufacturing of the static mixer for the mixer to tightly fit within the exhaust passage.
In one example, the issues described above may be addressed by a mixer comprising a hollow, annular ring having an internal exhaust passage, the ring including inlets located on a downstream inner surface and outlets located along an intersection between an upstream inner surface and the downstream inner surface adjacent to a throat of a venturi passage upstream of an SCR device, and a urea injector positioned to inject into the ring. In this way, the outlets may be fluidly coupled to a vacuum generated by flowing exhaust gas through the venturi passage.
As one example, vacuum generated by the venturi passage is supplied to an annular chamber created in the space outside the upstream and downstream inner surfaces and within the exhaust passage, where the vacuum supplied may draw exhaust gas through the inlets into the annular chamber. Exhaust gas in the annular chamber may mix with urea and/or exhaust gas from different regions of an exhaust passage. The inner ring surface may be shaped to form the venturi, and may fully separate exhaust in the central passage from the hollow interior, except for the inlets and outlet. The mixture of exhaust gas and urea flows into the inner central exhaust passage via the vacuum generated, where the mixture may merge with exhaust free of injected reductant. In this way, an entire exhaust flow through the exhaust passage may come into more contact with urea and improve an overall reduction of the SCR device.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.